1. Field of the Invention
The present invention relates to a distance measuring device and, in particular, to a distance measuring device which uses the principles of trigonometrical distance measurement and is used for automatic focusing in a camera.
2. Description of the Related Art
As a distance measuring system using the principles of trigonometrical distance measurement, conventionally, there has been known a system (hereinafter referred to as an active system) in which infrared light is onto an object to be photographed, light reflected from the object is received by a light receiving part (for example, a bisected SPD), and the distance of the object is measured in accordance with the light receiving centroid position of the light receiving part.
Also, there is also known another system (hereinafter referred to as a passive system) in which an image of an object is formed on two light receiving parts (CCD line sensors) by means of a pair of light receiving optical systems, and the distance of the object is measured in accordance with a difference between the phases of the two object images.
However, the above-mentioned active system has a problem in that, when the object is remote or has a low reflection factor, the infrared spot cannot be received by the light receiving part, which makes it impossible to measure the distance of the object. Also, in the passive system, there is a drawback that, when the object offers a low intensity of illumination or low contrast (that is, when the information on the object is insufficient), the distance to the object cannot be measured accurately.
On the other hand, the passive system may include a means which projects an auxiliary light onto the object when the object information is insufficient. In this case, however, an auxiliary light having a special pattern (for example, a striped pattern) must be projected so as to enable production of contrast. Also the fact that the object information is insufficient must be detected before the auxiliary light is projected, requiring a longer time to measure the distance of the object.